A rack unit is an Electronic Industries Alliance (EIA) measuring unit used to describe the height of a server. One rack unit is more commonly written as “1U”; similarly, two rack units are “2U” and so on. One rack unit is 1.75 inches (about 44.45 mm) high and 19 inches width, two rack units is 1.75*2 inches (about 89 mm) high and 19 inches width and so on. Therefore, according to the rule, in a server with N rack units, no matter how components are arranged within the server, the components must fit the height in a designated specification.
The space availability of a chassis of a 2U server available in the market is increased; however, the 2U server does not have expansibility. Typically, the chassis of the 2U server includes several motherboards, a power supply and signal lines therein, wherein the power supply is used to provide power for hosts, and the signal lines are used to electrically connect devices in the hosts. Each motherboard further includes a hard disk carrier, wherein several hard disks are disposed in the hard disk carrier. However, the chassis of the typical 2U server host only can contain several hosts due to the limitation of the line distribution of the signal lines, so that the space of the chassis cannot be effectively used for the expansion of other function modules, thereby preventing the work efficiency of the server from being further improved.